refactor: Enhance docs, add tests in Intersection

Author: Hardvan

src/main/java/com/thealgorithms/datastructures/hashmap/hashing/Intersection.java

@@ -1,27 +1,57 @@
 package com.thealgorithms.datastructures.hashmap.hashing;
 
-/*
- * this is algo which implies common mathematical set theory concept
- * called intersection in which result is common values of both the sets
- * here metaphor of sets is HashMap
- */
-
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
+/**
+ * The {@code Intersection} class provides a method to compute the intersection of two integer arrays.
+ * The intersection is defined as the set of common elements present in both arrays.
+ * <p>
+ * This class utilizes a HashMap to efficiently count occurrences of elements in the first array,
+ * allowing for an efficient lookup of common elements in the second array.
+ * </p>
+ *
+ * <p>
+ * Example:
+ * <pre>
+ * int[] array1 = {1, 2, 2, 1};
+ * int[] array2 = {2, 2};
+ * List<Integer> result = Intersection.intersection(array1, array2); // result will contain [2, 2]
+ * </pre>
+ * </p>
+ *
+ * <p>
+ * Note: The order of the returned list may vary since it depends on the order of elements
+ * in the input arrays.
+ * </p>
+ */
 public final class Intersection {
 
+    /**
+     * Computes the intersection of two integer arrays.
+     * Steps:
+     * 1. Count the occurrences of each element in the first array using a HashMap.
+     * 2. Iterate over the second array and check if the element is present in the HashMap.
+     * If it is, add it to the result list and decrement the count in the HashMap.
+     * 3. Return the result list containing the intersection of the two arrays.
+     *
+     * @param arr1 the first array of integers
+     * @param arr2 the second array of integers
+     * @return a list containing the intersection of the two arrays, or an empty list if either array is null or empty
+     */
     public static List<Integer> intersection(int[] arr1, int[] arr2) {
         if (arr1 == null || arr2 == null || arr1.length == 0 || arr2.length == 0) {
             return Collections.emptyList();
         }
+
         Map<Integer, Integer> cnt = new HashMap<>(16);
         for (int v : arr1) {
             cnt.put(v, cnt.getOrDefault(v, 0) + 1);
         }
+
         List<Integer> res = new ArrayList<>();
         for (int v : arr2) {
             if (cnt.containsKey(v) && cnt.get(v) > 0) {

src/test/java/com/thealgorithms/datastructures/hashmap/hashing/IntersectionTest.java

@@ -0,0 +1,76 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class IntersectionTest {
+
+    @Test
+    void testBasicIntersection() {
+        int[] arr1 = {1, 2, 2, 1};
+        int[] arr2 = {2, 2};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertEquals(List.of(2, 2), result, "Intersection should return [2, 2]");
+    }
+
+    @Test
+    void testNoIntersection() {
+        int[] arr1 = {1, 2, 3};
+        int[] arr2 = {4, 5, 6};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertTrue(result.isEmpty(), "Intersection should be empty for disjoint sets");
+    }
+
+    @Test
+    void testEmptyArray() {
+        int[] arr1 = {};
+        int[] arr2 = {1, 2, 3};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertTrue(result.isEmpty(), "Intersection should be empty when first array is empty");
+
+        result = Intersection.intersection(arr2, arr1);
+        assertTrue(result.isEmpty(), "Intersection should be empty when second array is empty");
+    }
+
+    @Test
+    void testNullArray() {
+        int[] arr1 = null;
+        int[] arr2 = {1, 2, 3};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertTrue(result.isEmpty(), "Intersection should be empty when first array is null");
+
+        result = Intersection.intersection(arr2, arr1);
+        assertTrue(result.isEmpty(), "Intersection should be empty when second array is null");
+    }
+
+    @Test
+    void testMultipleOccurrences() {
+        int[] arr1 = {5, 5, 5, 6};
+        int[] arr2 = {5, 5, 6, 6, 6};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertEquals(List.of(5, 5, 6), result, "Intersection should return [5, 5, 6]");
+    }
+
+    @Test
+    void testSameElements() {
+        int[] arr1 = {1, 1, 1};
+        int[] arr2 = {1, 1, 1};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertEquals(List.of(1, 1, 1), result, "Intersection should return [1, 1, 1] for same elements");
+    }
+
+    @Test
+    void testLargeArrays() {
+        int[] arr1 = new int[1000];
+        int[] arr2 = new int[1000];
+        for (int i = 0; i < 1000; i++) {
+            arr1[i] = i;
+            arr2[i] = i;
+        }
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertEquals(1000, result.size(), "Intersection should return all elements for identical large arrays");
+    }
+}